Offset voltages in DC-coupled amplifier circuits are balanced in various ways.
A conventional method is to balance the respective amplifier circuit once upon start-up by applying a compensation voltage to its input. This does not take into account any subsequent changes in offset voltage due to thermal drift or the like.
Furthermore, methods are known which permit two or more offset-balancing operations by means of an automatic control system.
In this connection, reference is made, for example, to an article by T. F. Collura et al., "Automated offset compensation for DC biopotential measurements", published in "Behavior Research Methods, Instruments & Computers", 1990, 22(1).
The levels of the received signals may vary from section to section (from time slot to time slot). That is the case particularly in TDMA radio receivers, which, like the GSM standard, have a time-slot allocation to fixed (time) channels, so that according to the different transmission conditions in the individual channels (coming from different transmitters), levels occur which vary widely from time slot to time slot.
Offset correction methods for input signals with levels changing section by section, as occur particularly in TDMA radio receiver, are presented, inter alia, in U.S. Pat. No. 4,873,702 and in Patent Application PCT/US90/05358.
In both methods, a time interval defined in the respective section and having a known average value ("preamble signal" and "DC compensation signal", respectively) is used to determine the correction value. This value, according to the principle of the above methods, is not affected by the variations of the input signal levels. For input signals having no such time intervals, the above methods cannot provide meaningful offset correction. If such time intervals are available, however, their evaluation for determining a suitable correction value will be particularly problematic if the incoming signal is significantly distorted within a time-slot duration.
That is the case, inter alia, with fading or multipath as occur in radio systems as specified in the GSM standard, for example. These effects also change the above-mentioned time intervals ("preamble signal" and "DC compensation signal", respectively) so that correction values determined therefrom may differ considerably from the actual offset value to be compensated, and thus may result in miscompensation.